1. Field of the Invention
The present invention is directed to a process for the production of highly degradation resistant rolling mill cast rolls as well as to the rolls produced through this process.
2. Background Art
As is known by those skilled in the art, the rolling mill rolls are tools used in the rolling process, by means of which plain or long metallic material products are shaped, predominantly steels. Like any shaping tool, it is the rolling mill roll that contacts directly the shaped product, thus leading to the continuous and increasing degradation of the surface thereof during its use as well as the consequent need to recover the initial condition of this surface after a certain period of use. Such recovery is carried out by removing the deteriorated surface layer through machining, thus requiring the shutdown of the rolling mill to extract the degraded roll and replace same with a new or already repaired roll.
Therefore, the quality and the productivity of the rolling process are intimately connected to the service performance of the rolling mill rolls, since:
(a) the quality of rolled products is mainly determined by the precision and repeatability of the shape thereof, that reflect directly the geometry and the condition of the surface of the rolls;
(b) the productivity of the rolling mill is determined in part by the uninterrupted service time that the rolling mill roll can withstand, keeping the quality of the rolled product above the minimum level set (the minimization of the shutdowns to exchange the rolls, made possible by rolls that are more resistant to degradation, implying directly in the increase of the productivity of the production line.
In the final phase of the process of hot-rolling strips, a set of in line rolling modules (“roll stands”) that is called finishing train is used. The amount of roll stands may vary as a function of the design and the conception of the rolling mill itself.
As there is a difference of degradation factors between the first roll stand and the last roll stand, different materials of rolls are also used.
The degradation of the rolling mill rolls happens to be a process characterized by the simultaneous action of distinct wear modes: abrasion, oxidation, adhesion and thermal fatigue. However, it is known that one or two of such wear modes in each roll stand of the rolling mill train are predominant: in the first roll stand, where the shaping takes place at temperatures of the order of 1,000° C., the thermal fatigue and oxidation phenomena act more intensely; in the last roll stand, where the shaping takes place at temperatures of the order of 700° C., the abrasion and adhesion are predominant.
Another drawback found in the finishing train provided with traditional rolls is the fact that, together with the gradual degradation of the surface of the rolling mill roll, unforeseen damages caused by operational incidents of the finishing train, such as the adhesion of the rolled strip on the roll (“sticking” or “welding”) and the unstable propagation of surface or subsurface cracks on the roll, cause the shutdown of the operation of the rolling mill for removing the roll. Two operational characteristics of the last roll stands of the finishing train make the rolls that work thereon more susceptible to such damages:
(a) low rolling temperature (that determines the low kinetics of the oxide formation on the surface of the roll, what compromises the performance thereof as a protection against the adhesion);
(b) little thickness of the rolled strip.
The rolling mill rolls designed to the finishing trains, in most of the cases, are bimetallic cast components that are comprised of an “external shield” made of a wear resistant alloy and a “core” made of nodular or gray cast iron. The process that is commonly used for the production of such bimetallic rolls is centrifuged casting in a metallic mold: the material of the shield is poured into the mold and, due to the action of the centrifugal force, it is distributed uniformly over its inner surface, forming an outer layer (or shield) with a thickness between 40 and 120 mm; after the solidification of the shield, the material of the core is poured into the same mold still rotating, thus filling up same. When in contact with the inner surface of the already solidified shield, the material of the core recasts a low volume of the shield (about 10 mm along the whole inner surface thereof), thus creating a metallurgical link between the shield and core that is called interface.
For the first roll stands, where the oxidation and thermal fatigue factors are predominant, rolls with a working layer (shield) made of white high-chromium cast iron are prevalent, as well as multi-component white cast irons, also called high-speed steels; therefore, such rolls have a service performance of the order of at least twice the one observed for the chilled cast iron that is usually used in the last roll stands, where the abrasion and adhesion factors predominate, and the strip adhesion phenomenon is frequent too.
Thus, another disadvantage found in the finishing trains (last roll stands) provided with traditional rolls is a disparity of the need to exchange rolls between the first roll stand and the last roll stand, thus compromising a possible prolongation of the working time, and it is clear that if said disparity could be favorably eliminated the number of roll exchanges of the last roll stands would be reduced and the productivity of the rolling mill would be increased.
While the development of alloys for the shield of rolls designed to the first roll stands of the finishing train has been intense in last the 20 years, in the shield of rolls designed to the last roll stands a Fe—Cr—SiNi—C alloy that is called chilled cast iron has been used for more than 40 years. The microstructure of this material has a tempered martensite die with a precipitation of secondary carbides M3C and interdendritic web of eutectic carbides M3C (volumetric fraction of approximately 25%), besides lumpy or nodular graphite that also is interdendritic (volumetric fraction of approximately 3%). This microstructure is the resultant of the balance of the characteristics of the alloy elements in the chemical composition: silicon and nickel are graphite producing components, chromium is a strong carbide producing component and, additionally, nickel also determines the hardenability needed to prevent the formation of perlite in the cooling after the casting.
The long useful life of chilled cast iron that are applied to rolls of finishing roll stands is attributed to the fact that its microstructure has represented, until the present date, the best compromising solution between the abrasive wear resistance that is promoted by the die and eutectic carbides, and the adhesion resistance and unstable propagation of microcracks that are promoted by the graphite. It should be understood that the graphite acts as a lubricant in the roll/rolled strip interface (diminishing the adhesion) and decreases the concentration of stress at the end of microcracks, thus diminishing the propagation rate thereof.
A few substantial developments had been carried out in the product line/process of rolls designed to the last roll stands of the hot rolling mill of strips. Among the most expressive according to this same conception of material, that is, the chilled cast iron, we may cite the ones that include additions of alloy elements for the formation of hard second phase particles that only comprise dispersed fine carbides. Changes have also been made to the process of solidification in order to attain more refined structures.
The efforts of the manufacturers to satisfy the demand of the users of such rolls as to a higher abrasive wear resistance, while keeping the other performance characteristics, are based on the use of strong alloy elements that form eutectic and/or secondary carbides harder than carbide M3C, such as, for example, M7C3 and MC type carbides.
However, it was found that this is counter-productive for promoting a reduction or suppression of the graphite formation and, consequently, diminishing the adhesion resistance and the unstable propagation of microcracks; that is, the resulting compromising solution has not implied an increased productivity of the rolling mill train.
Patent PCT/U51996/09181 discloses a process for the production of chilled cast iron with the addition of between 0.3% and 6% niobium by weight and with a service performance that supersedes that of chilled cast iron. The presence of niobium promotes the formation of isolated particles of primary carbide NbC with an increased hardness, without compromising the graphite formation, thus increasing the abrasive wear resistance and keeping the adhesion resistance and the unstable propagation of microcracks (the formation of carbide NbC takes place at temperatures higher than that of graphite and, therefore, there is no competition between same).
However, the addition of niobium is practically of the order of 0.6%, therefore, higher contents imply the need to substantially increase the pouring temperature while roll is being cast, thus making the process unworkable. With this addition level, the increase of the abrasive wear resistance is limited, since the volumetric fraction of eutectic carbide NbC in the microstructure is limited to values lower than 1%, besides the absence of precipitation of secondary carbide NbC in the die.
In addition to such attempts, only a few rolling mills use multi-component white cast iron in the last roll stands that are obtained either by a centrifugal casting process or a CPC (“continuous pouring cladding” process or, in vernacular language, “coating through continuous casting”) However, a problem that such rolling mills face is that the composition of this type of alloy does not contain graphite, thus compromising the adhesion resistance.